The present invention relates in general to the treatment of fusible materials such as solder, and in particular to a new and useful process for treating the surface of solid fusible material.
In the manufacturing of electronic equipment, components are usually mounted to circuit boards by soldering. It has been known for many years to supply the required solder during a soldering process from a reservoir containing liquid solder. Liquid solder is available from the reservoir in large amounts and is supplied using known techniques such as in a liquid wave.
Usually different techniques are required for supplying different amounts of solder for connecting the various components to a board. One typical example is the supplying of solder in the form of a paste which is printed to cover defined areas of the board. The three dimensional shape or height of the solder determines the volume of solder at any particular location on the board.
It is critical to supply the correct amount of solder within a specified tolerance, particularly for areas which will not tolerate excess solder. One such situation is where flat ribbons acting as conductors from a typical flatpack component is to be solder mounted to flat conductors on the board with almost 100% coverage. If too much solder is present, the solder will bridge adjacent conductors and form a short circuit.
If too little solder is available an imperfect solder joint will result. Standard procedures for placing solder create wide ranges of tolerances which are often caused by different shaped pads as well as different components to be mounted to the board.
The required layers of solders can also be plated to pads on the board using a known galvanic process. To maintain high quality in this plated solder, the solder is usually fused in a separate thermal process prior to the mounting process. These processes are performed over the entire board. This creates a wide range of tolerances. Other known processes for solder plating also create wide ranges of tolerances which often cannot be accepted for manufacturing purposes. Another disadvantage of such processes is that once the solder on the pad fuses, it loses its flat shape and becomes rounded due to surface tension of the liquid solder. In particular with small dimensions, it is very difficult to position leads of a component on such non-flat surfaces because the leads may easily move to the sides of the rounded shapes. This may move the entire component from its desired location and create electrical short circuits. There is currently almost no known procedure which reduces the wide range of tolerances in the solder provided on circuit boards. This effects the entire yield during the manufacturing process which usually requires excessive reworking of unacceptable parts.
The metallic solder which is used to form a solder joint between two or more metallic parts, must, in its liquid state, form intermetallic phases with the material of the metal parts. This requires the relevant surfaces to be metallically clean. In particular, this means that the parts must be free of oxide layers.
As a practical matter, this requires the application of flux material to reduce contamination at the surfaces. At higher temperatures, fluxes are chemically activated to reduce contamination at the metallic surfaces for improving the joining process. Flux also protects the surfaces for a limited time period during the joining process to avoid contamination by newly formed oxides.
For most applications, it is not easy to supply the correct amount of flux and to keep the flux in the areas it is needed during the joining process. If excessive force is applied to the parts which are to be joined together, the flux may be squeezed out from between the parts leaving very little flux in the areas where it is needed. Further flux generally moves from areas of higher temperature to areas of lower temperature, further frustrating the joining process.
These problems are particularly evident during so called pulsed-hotbar soldering. Pulsed-hotbar soldering uses a tool which is heated by a pulse of energy to join parts which are held against each other by force applied to the heated tool. The squeezing of flux out from the joining area where it is needed is a typical feature of this process such as when flat ribbons of an integrated circuit package are being connected to the pads of a printed circuit. This problem limits the applicability of pulsed-hotbar soldering techniques in this environment.
Some known processes involving brazing with hard solder, supply flux together with the solder in a solid form. The required amount of solder is thus available during the fusing process. Soft soldering processes using tin-solder alloys are also known where the flux is provided inside a solder wire. The solder wire is applied to heated surfaces to be joined while the necessary amount of flux is supplied automatically.